Date: Thu, 31 Jul 2003 21:43:27 -0600 Subject: PCI bus speed? From: Mike Kauspedas <[EMAIL PROTECTED]> Message-ID: <[EMAIL PROTECTED]>
Is the PCI bus the same speed as system bus on a PM 7500? Mine is clocked at 50MHz right now. I assume the PCI is 33MHz. Fairly new to Macs, just curious.
The PCI bus speed is 33 MHz, no matter which CPU card you install. The CPU/memory bus is a different bus from the PCI bus. Hence, the two busses can have different speeds.
On the x500/x600 series of machines, the PCI bus speed is set by a 33 MHz osciallator soldered to the motherboard. The only way to change the PCI bus speed would be to replace that oscillator (or get wild with the clock buffer chip that divides the signal amongst the PCI slots and Bandit chip(s)).
The Bandit chip (343S0020) "bridges" between the CPU/memory bus and the PCI bus. On one side (literally, the CPU bus pins are on one side of the chip and the PCI bus pins are on the other, both wrapping around the sides a bit) the pins of the Bandit chip connects to the CPU bus. On the other side, the pins are connected to the PCI bus. The stuff inside the chip translates transactions on the CPU/memory bus into PCI transactions and vice versa.
On an irrelevant note, the Bandit chip is just like another device on the PCI bus. It simply has a very special function. Arbitration (which device talks when) and priority (what happens when two want to talk at the same time) is handled by a separate arbitration chip. That's a little 28 pin square PLCC chip marked 343S0182 or maybe 343S082. The 7200 seems to have an earlier version of the chip.
On the CPU/memory side, bus arbitration is handled by the Hammerhead memory controller (343S1190).
Jeff Walther
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